Removal of chromium from plating baths and electroplating process



mte States patent REMOVAL OF COMIUM FROM PLATING EATHS ANDELECTROPLATING PROCESS Erich Laue, Watertown, Conn, assignor toMacDcrmid Incorporated, Waterbury, Comm, a corporation of Connecticnt NoDrawing. Application November Serial No. 696,058

16 Claims. (Cl. 204-44 purities present a frequent problem sinceindustrial platers generally employ the same'racks for plating articlesto produce successive deposits of copper, nickel and chromium. Thus,chromium is introduced from the racks to the copper plating bath, forexample. Whileit might be supposed that the minute quantities of themetal which are transferred from the racks to the bath would have littleeffect upon the elficiency of the copper plating operation, it has beenfound that the presence of chromium in concentrations of only lto 10 p.p. m. CrO causes low current density dullness, etched patterns andstaining. At 10 to 20 p. p. m. CrO blistering and skip plate results,and at still higher concentrations copper plating is in many instancescompletely inhibited. The exact chromium level at which these effectsoccur depends in part upon the presence in the bath of addition agentswhich are often employed, such as brighteners, secondary complexers orchelating agents'and their concentration inthe bath.

In many alkaline plating baths salts of organic'hydroxy acids, a classof compounds whichl term secondary complexers are used as additionagents. Many copper cyanide baths, for instance, contain Rochelle salts.These compounds have a beneficial efiect on the operation of the anodes.In other applications they are required to keep metallic addition agentsin solution. agents such as ethylene-diamine tetracetate may be used ina similar manner.

In alkaline plating baths which contain no secondary complexers orchelating agents, chromium impurities can be removed by reduction to thetrivalent state followed by precipitation and filtration as .Cr( H Ifsecondary complexers or chelating agents are present, as is very oftenthe case, trivalent chromium is tied up and kept in solution and theaforementioned simple precipitation is no longer possible. Since thedeleterious effect of chromium upon the electroplating process is causedby chromium in the hexavalent state, simple reduction to 'Cr+ eliminateschromium trouble temporarily. Unfortunately, the benefit is short livedsince in the'course of the plating operation trivalent chromium isre-oxidized and must be reduced anew. It can be seen that this method ofhandling the chromium impurities requires frequent corrective action,and by-products from the reducing agentemployed tend to accumulate inthe bath to a detrimental level.

Effects of chromium contamination can be alleviated to some extentthrough addition of secondary complexersor chelating agents. However,asmore-chromium enters theg Patented May 6,1958

bath from the racks holding the work, more chelating agent must beadded, and eventually these additions interfere with plating andultimately the bath becomes inoperable.

Accordingly, it is an object of the present invention to completelyovercome chromium contamination problems in alkaline plating bathscontaining secondary complexers or chelating agents. It is a furtherobject to permanently remove chromium impurities from the plating bathby a process which will not interfere with the main constituents of thebath. It is a still further object of the present invention to employreagents which do not interfere with plating and which can easily beremoved from the bath.

In accordance with the present invention, any hexa- I valent chromium,present in the bath is first adjusted to the trivalent state. While itis possible that all or a large part of the chromiummay be present inthe trivalent state,I preferably initially add to the bath an agentcapable of reducing. hexavalent chromium to the trivalent state insuflicient quantity to reduce all the chromium even if all of it werepresent as hexavalent chromium. Next I add to the bath, a substituted'anthraquinone, namely: l,2-dihydroxy anthraquinone (alizarin), a ringsubstituted 1,2-dihydroxy anthraquinone capable of forming a chromiumlake in the plating bath, or mixtures of these 1,2-dihydroxyanthraquinones. Following formation of the chromium lake, suflicientactivated carbon is added to the bath to adsorb any colloidal lake andsubstantially all of the excess substituted anthraquinone added. Thecarbon together with the chromium lake is then separated from the bathas by filtration. The thus treated bath is then substantially free fromchromium.

Surprisingly, chromium lake formation is possible in the bath with onlya few of the substituted anthraquinones. For example, if the hydroxygrouping is other I than 1,2, the compounds are ineffective. This leavesa Chelating relatively small class of compounds which includes 1,2-dihydroxy anthraquinone (alizarin) and the ring substituted1,2-dihydroxy anthraquinones. Among ring substituents which may becontained in these compounds are OH, S0 H, etc. The following which arecommercially available perform well: the sodium salt of ali'zarin3-sulfonic acid- (alizarin red), 1,2,3-trihydroxy anthraquinone(anthraga-llol), 1,2,3,5,6,7-hexahydroxy anthraquinone (rufigallol), aswell as a mixture of anthragallol and rufigallol (alizarin brown).Alizarin is preferred be cause of its more attractive price.

Chromium is easily removed with relatively small quantitles of thesubstituted anthraquinone. For example, a 10 fold molecular excess isoften entirely satisfactory. However, in the presence of chelatingagents or secondary complexers which hold the chromium more tightlylarger excesses are required. Of course, unusually high chro-, miumconcentrations or the presence" of large quantities of the secondarycomplexers or chelating agents necessitates larger substitutedanthraquinone additions. Generally, the upper limit of application willbe dictated by cost only. Chromium contaminations normally occurring inthe industry can usually be handled with quantities rang-,

ing from 5 to 30'ounces of'solid substituted anthraquinone 4 solution.

Chromium has thus been successfully removed from 3 cyanide bathscontaining a variety of secondarycomplexers including tartrates andgluconates, and from baths containing chelating agents, among which wereethylene-.

diarnine tetracetate, triethylene tetramine and Yersene T. l

- in a high degree of complexity. For example, "chromium The'process ofthe present invention is applicable to i all alkaline baths whichcontain the metals to be plated was eliminated from a" silver cyanidebath containing potassium tartrate, as well as from a brass platingsolution containing Rochelle salts. n the other hand, the presentprocesswas ineffective for removing chromium from a'copper pyrophosphatebath,because, it is believed, inthis' bath considerablequantities ofuncomplexed'copper were available andconsumedthe1,2-dihydroxy'arithraquinone employed leaving none for thechromium impurity. Thus, it is seen that the'plating metal or othermetal desirably present must be in a high degree of complexity so thathis incapable of forming a complex with the substituted anthraquinoneflh As noted earlier, where allor part of the chromium contamination ispresentin thehexavalent state, it must be; reduced before orsimultaneously with'addition'of the substituted anthraquinone. Excellentresults 'were' obtainedwith the use; of a premixed reagent containingboth -thefi substituted -anthraquinone and the reducing.

agent in concentrated aqueous solution. Whileseveral' agents areavailable for-reducing chromium in a plating bath, one often employed issodiumdithionite, the hydrosul fite of commerce, which performsexceedingly well. Stannous compounds such as stannous sulfate-may alsobe used. However,- stannous tin will interfere with copper cyanideplating processes,-'and accordingly when a stannous salt is used as thechromium reducing agent, followingremoval of chromium lake from thebath, a small amount of hydrogenperoxide is addedto oxidize anyexcessstannous compound to stannic tin, which does not-affect plating. I Y

Following formation of the chromium lake, sufiicient' activated carbonis added to the bath to pick up colloidal chromium lake and adsorbexcess substituted anthraquinone. For the sake of economy several smallcarbon additions may be made. However, removal of excess substitutedanthraquinone is not absolutely necessary. It does not interfere withthe plating process. I The carbon and lake are removed by filtration. I

The chromiumremoval process of the present invention is.highlyeflicient, and requires no adjustment of the pH. of1the-bath,which1is usually well above a pH of 9 and in thevicinity of 13.Furthermore, the present process can be carried out over a wide range oftemperature,

although elevated temperatures give best results since they favor; thecolloidal reactions involved in the overall chromium removal,v Thepreferred operating temperature is about 160 to 212 F.

The following non-limiting example. illustrates the process of ,thepresent invention as applied to ,a copper cyanide plating bath.

In addition tothe alkali and cyanideconstituents, a plating solutioncontained 4.2 ounces 'per gallonof copper. metal and 6 ounces per gallonof potassiumttartrate as a secondary complexer. The bath had a. pH ofabout 13. Plating in this bath produced etched patterns. and skip plateinplaces. An analysis showed a chromium contamination of 20p. p. m. CrO.1. In order. toremovechromium 100 gallons of the above solution werefirst heated to 200 F. Areagent was prepared as follows; to; 3 gallonsofcold water containing 7 ouncesof solidcaus tic potash, 14 ounces ofalizarin were. added, and the mixture was then agitated until, thecomponents completely dissolved. In another vessel 3 ounces, of stannoussulfate were dissolved in 1 gallon of water. Tofthis solution were added12 ounces of solid caustic potash [and thelmixture was stirred tocomplete dissolution. The reducing agent solution was stirred. into thealizarin solution and the'resulting solution was I added to theghotplating solution. The reaction mixture was kept mildly agitated at 200FL for 15 minutes. Then' flui was. 95% hY IQEQ ml de. Q llw l wi h:

water were then added to the bath to oxidize excess stannous tintherein.

The resulting purified copper cyanide plating bath produced excellentplate completely free of all chromium contamination eifect. Analysisshowed that now less chromium was present than could be detected by theanalytical method, namely, less than 0.5 p. p. m. CrO

What is claimed is:

1. A process for removing trivalent chromium contaminants from a metalcyanide plating bath, which comprises adding thereto a substitutedanthraquinone capable of forming a chromium lake therein selected fromthe group consisting of 1,2-dihydroxy anthraquinone, ring substituted1,2-dil1ydroxy anthraquinones and mixtures thereof, contacting the thustreated bath with activated carbon and then. separating the carbon,chromium lake and adsorbed substituted anthraquinone from the bath.

2. A process for removing chromium contaminants from a metal platingbath containing a stable metal cyanide complex, which comprisesadjusting any chromium of higher valence present in the bath to thetrivalent state, adding a substituted anthraquinone capable of forming achromium lake therein selected from the group consistingof 1,2-dihydroxyanthraquinone, ring substituted 1,2-dihydroxy anthraquinones andmixtures thereof, contacting the thus treated bath with activated carbonand then separating carbon, chromium lake and adsorbed substitutedanthraquinone from the bath.

3. A process according to claim 2 whereinthe plating metal is selectedfrom the group consisting of copper, silver and brass. Y

4. A process according to claim 2 wherein the substituted anthraquinoneis 1,2-dihydroxy anthraquinone.

5. The process according to claim 2 wherein the substitutedanthraquinone is 1,2,3-trihydroxy anthraquinone.

6. A process according to claim 2 wherein the substitued anthraquinoneis l,2,3,5,6,7-hexahydroxy anthraquinone.

7. A process according to claim 2 wherein the substituted anthraquinoneis a mixture of 1,2,3-trihydroxythraquinone and l,2,3,5,6,7-hexahydroxyanthraquinone.

, 8. A process according to claim 2 wherein the substitutedanthraquinone is the sodium salt of alizarin 3-sulfonic acid.

9. A process for removing chromium contaminants from a metal cyanideplating bath containing an addition agent of the group consisting ofsecondary complexers and chelating agents, which comprises reducing anyhexavalent chromium present to the trivalent state, adding to the bath asubstituted anthraquinone capable of forming a chromium lake thereinselected from the group consisting of 1,2-dihydroxy anthraquinone, ringsubstituted 1,2-dihydroxyanthraquinones and mixtures thereof, contactingthe thus treated bath with activated carbon and then sep aratingthecarbon, chromium lake and adsorbed sub stituted anthraquinone fromthe bath.

10. A process according to claim 9 whereinthere is employed a stannouscompound as reducing agent.

11. A process according to claim 10 wherein following separation of thecarbon and chromium lake. any excess stannous compound is oxidized.

12. A process according to claim 11 wherein following removal of carbonand chromium lake hydrogen peroxide is added to the bathin amountsutncient to oxidize excess stannous compound.

.' l3.'A process for removing chromium contaminants from a. cyanideplating bath containing metal selected from the group consisting ofcopper, 'silver and brass and containing an addition agent of the groupconsisting of secondary complexers, and chelating agents, whichcomprises reducingany hexavalent chromium present to the trivalentstate,adding to the bath a substituted anthraquinone capable of forming achromium lake therein se lected from the group consisting of1,2-dihydroxy anthraquinone, ring substituted 1,2-dihydroxyanthraquinones and mixtures, thereof, contacting. the .thus. treated.bath:

with activated carbon and then separating the carbon, chromium lake andadsorbed substituted anthraquinone from the bath.

14. A method of electroplating metal from a cyanide plating bathcontaining chromium impurities to produce an electrodeposited plate freefrom surface stain, etched patterns and blisters caused by said chromiumimpurities, which comprises adjusting any chromium of higher valencepresent in the bath to the trivalent state, adding to the bath asubstituted anthraquinone capable of forming a chromium lake selectedfrom the group consisting of 1,2-dihydroxy anthraquinone, ringsubstituted 1,2-dihydroxy anthraquinones and mixtures thereof,contacting the thus treated bath with activated carbon, then separatingcarbon, chromium lake and adsorbed substituted anthraquinone from thebath, and electroplating said metal from the resultingsubstantiallychromium-free bath.

15. A method as set forth in claim 14 wherein the metal is selected fromthe group consisting of copper, silver and brass.

16. A method of electroplating metal from a cyanide .6 plating bathcontaining chromium impurities and an' addition agent of the groupconsisting of secondary complexers and chelating agents to produce anelectrodeposited plate free from surface stain, etched patterns andblisters caused by said chromium impurities, which comprises adjustingany chromium of higher valence present in the bath to the trivalentstate, adding to the bath a substituted anthraquinone capable of forminga chromium lake selected from the group consisting of 1,2-dihydroxyanthraquinone, ring substituted 1,2-dihydroxy anthraquinones andmixtures thereof, contacting the thus treated bath with activatedcarbon, then separating carbon, chromium lake and absorbed substituedanthraquinone from the bath, and electroplating said metal from theresulting substantially chromium-free bath.

References Cited in the file of this patent Beckwith: ProceedingsAmerican Electroplaters Society, vol. 28 (1941), pages 543-549.

1. A PROCESS FOR REMOVING TRIVALENT CHROMIUM CONTAMINANTS FROM A METALCYANIDE PLATING BATH, WHICH COMPRISES ADDING THERETO A SUBSTITUTEDANTHRAQUINONE CAPABLE OF FORMING A CHROMIUM LAKE THEREIN SELECTED FROMTHE GROUP CONSISTING OF 1,2-DIHYDROXY ANTHRAQUINONE, RING SUBSTITUTED1,2-DIHYDROXY ANTHRAQUINONES AND MIXTURES THEREOF, CONTACTING THE THUSTREATED BATH WITH ACTIVATED CARBON AND THEN SEPARATING THE CARBON,CHROMIUM LAKE AND ADSORBED SUBSTITUTED ANTHRAQUINONE FROM THE BATH.